In recent years, in association with the development in portable equipment such as personal computers and mobile phones, the demands for batteries as power sources thereof have been increased. Such batteries for use in the foregoing applications are required to be used at normal temperature and are expected to have a high energy density and excellent cycle characteristics.
In order to meet this expectation, new active materials with high capacity for use in a positive electrode or a negative electrode have been developed. The most promising among these is a battery in which a simple substance of silicon (Si) or tin (Sn), or alternatively, an oxide or alloy thereof capable of providing an extremely high capacity is used as a negative electrode active material.
However, for example, in a battery in which a material containing silicon is used as a negative electrode active material, there is a problem in that the deformation of the negative electrode occurs in association with repeated charge and discharge. Specifically, during charge and discharge, insertion and extraction of lithium (Li) causes the negative electrode active material to expand and contract greatly. The repeated charge and discharge, therefore, causes the negative electrode to warp severely, which may result in wrinkles on the current collector, and a cut in the current collector. Moreover, a space is produced between the negative electrode and the separator, causing the charge-discharge reaction to proceed unevenly. As a result, the battery performance is deteriorated.
In order to solve the problem as described above, one proposal suggests that the negative electrode active material be provided with a space for relieving expansion stress of the active material. This proposal intends to reduce warps or waves of the negative electrode and suppress the deterioration of the cycle characteristics.
For example, Patent Document 1 suggests forming columnar particles of silicon on a current collector.
Patent Document 2 suggests arranging an active material capable of alloying with lithium regularly on a current collector in a predetermined pattern.
Patent Document 3 suggests forming a thin film electrode made of silicon, tin, or the like on a current collector with projections and depressions and then flattening the projections and depressions to form reticulate cracks on the thin film.
Patent Document 4 suggests allowing columnar particles composing a negative electrode active material to be slanted with respect to a direction normal to the surface of a current collector.    Patent Document 1: Japanese Laid-Open Patent Publication No. Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-127561    Patent Document 3: Japanese Laid-Open Patent Publication No.    Patent Document 4: Japanese Laid-Open Patent Publication No.